The present invention relates to an electrical circuit arrangement for producing a low power rectified low voltage from an AC line voltage.
In the present application, low power is understood to mean powers below 5 watts.
For this purpose, power supplies with 50 Hertz transformers are mostly used. These are large, heavy and are not very efficient. In particular, they consume a lot of energy even when their load is switched off.
As an alternative, switched-mode power supplies may be used. Conventional switched-mode power supplies consist of merely a single stage, generally a flyback converter.
If one wishes to operate such switched-mode power supplies at the 230 volt line voltages conventional in Europe, a relatively expensive 600 volts switching transistor is necessary.
In many applications, the high 230 volts input voltage additionally leads to high switching losses.
In addition, the high input voltage requires a transformer with a high main inductance. Consequently, only transformers with large core cross sections and/or large numbers of turns may be used.
A disadvantage of such flyback converters is also the sharp increase in the output voltage and the output power in the event of a regulation fault.
In order to protect the load connected to the low voltage side even when the output voltage increases erroneously sharply, an additional protective circuit has to be installed.
These disadvantages make switched-mode power supplies in the power range below 5 watts more expensive to produce than a 50 Hertz transformer power supply.
A transformer-less power pack is known from EP 0 229 950 B1 which has a capacitive input stage. Such capacitive input stages are also found in U.S. Pat. No. 3,355,650 and in DE 40 37 722 C1.
Such capacitive input stages do not generally offer any line isolation and are therefore not suitable for operating electrical current consuming apparatus which has to be operated with isolated low voltage.
Moreover, where such capacitive input stages are used for very low output voltages, a relatively large input capacitor is necessary.
If it is further assumed that such small power supplies are frequently in operation for days, months or years and for a considerable proportion of that time are kept in no-load operation (stand-by), taken together all current conventional small power supplies constitute a considerable cost factor with regard to total current consumption.
It is therefore an object of the present invention to propose an electrical circuit arrangement for producing a low-power rectified low voltage from an AC line voltage which allows isolated supply of electrical current consuming apparatus which has to be operated in the home and industry at low voltage.
This object is achieved by the invention having the features of claim 1.
Thus, a low-cost low-power power supply (e.g. 2 watts) is provided which transforms electric energy from the domestic electricity supply system of for example 230 volts or 110 volts into potential insulated voltage of for example 6 volts.
A decisive feature of the invention is the combination of two circuit blocks, of which the first circuit block constitutes a capacitive input stage, which generates an intermediate circuit voltage of limited voltage and current, which serves as input voltage for the second circuit block.
The second circuit block comprises first of all essentially an alternating mode operated, asymmetric transistor half-bridge for producing an AC voltage typically of approximately 50 volts with a DC component which typically amounts to around half the intermediate circuit voltage, wherein the AC voltage at this relatively low voltage level is stepped down to the low voltage via a pair of transformer turns and then rectified.
An essential feature of the invention is the successful integration of a capacitive input stage and a downstream half-bridge with subsequent insulation of output and input potential.
In this way, the capacitive input stage reduces the input voltage, rectifies it and limits the input power.
The intermediate circuit voltage produced in this way is limited to a maximum value.
Since the high-frequency line isolation transformer is arranged in the second, downstream circuit block operated at reduced voltage, small, light and small transformers may be used.
By limiting the intermediate circuit voltage, the maximum output values too are limited, so that savings may be made in overvoltage protection at the output of the power pack, i.e. on the low-voltage side.
Furthermore, on the secondary side only one output diode is necessary. Nonetheless, a two-way rectifier may also be provided.
Another essential feature of the invention is that the transistor half-bridge is driven in alternating mode.
Feedback of the output variable of the power supply to the alternating mode clock generators or switching transistors respectively, is not necessary in principle, since even with a fixed duty cycle of the alternating mode clock generators or switching transistors respectively, it is possible to achieve a very good output characteristic. This is an advantage over other switched-mode power supply topologies. The duty cycle p of the half-bridge or of the alternating mode clock is here defined as follows: p=t on/T, wherein t on is the on-time of a particular transistor specified in more detail in the examples of embodiment and T is the duration of a switching cycle. This will be addressed in more detail with reference to the examples of embodiment.
Tests have shown that the maximum voltage arising is typically only 20% above the nominal voltage and the maximum current is typically only 60% above the nominal current.
Nonetheless, feedback is possible, if the output needs to be better regulated.
Moreover, it may be ensured that at the instant of closing of each transistor substantially no voltage is applied thereacross, such that only little effort is required to achieve EMC filtering and in addition the power loss in the transistors is markedly reduced. This effect, however, does not absolutely have to be achieved at all operating points. It may be achieved as a function of a plurality of parameters for all working points of interest.
By operating the second circuit block with the voltage-reduced intermediate circuit voltage, the further advantage is achieved that all semiconductors of the second circuit block together with their drive elements may be integrated with a low-voltage IC. These are understood to mean integrated circuits whose operating voltage is typically below 100 volts. Since, in addition, none of the semiconductors of the first stage are exposed to any higher voltage than the intermediate circuit voltage, these may likewise be integrated with the same IC, but the rectifier diodes thereof often cannot because of their overcurrent bearing capacity.
Furthermore, in addition to unregulated operation, which is suitable for many applications, regulated operation is also possible at low expenditure.
Regulated operation is preferably effected via the duty cycle of the half-bridge. The duty cycle of the half-bridge allows fixed-frequency operation and thus simple EMC filtering. Moreover, the drive signals for the half-bridge signals may be produced with markedly less expenditure in the case of fixed-frequency operation than, for example, regulation via the intermediate circuit voltage. By effecting regulation via the duty cycle of the half-bridge, also the transmission of fluctuations in the intermediate circuit voltage, the output voltage, due to the 50 Hz input signal and due to regulation of the intermediate circuit voltage, may be prevented, so that voltage fluctuations which arise may always be corrected.
This is addressed in greater detail in the examples of embodiment.